1. Field of the Invention
This invention relates to fluid coupling devices in general, and more particularly to a temperature responsive fluid coupling device which controls the rotation of a cooling fan of an internal combustion engine in response to changes of ambient temperature.
2. Description of the Prior Art
Fluid coupling devices of this type control the amount of fluid conducted from a reservoir chamber to a working chamber in response to changes of engine ambient temperature in order to prevent the engine from being overcooled and to reduce the horsepower loss of the engine. There has been introduced a fluid coupling device which controls transmitting torque in three steps by means of selectively opening and closing a fluid return opening by a valve plate.
A conventional fluid coupling device of this type which controls transmitting torque in three steps in response to changes of ambient temperature comprises basically the following construction. That is to say, the fluid coupling device includes an input coupling member having a rotatable rotor, an output coupling member rotating relative to the input coupling member, a dividing plate dividing an interior of the output coupling member into a reservoir chamber for a viscous shear fluid and a working space accommodating the rotor, a plurality of operating chambers formed between a side surface of the rotor and an inner surface of the output coupling member, a plurality of fluid return openings formed through the dividing plate to provide the fluid to the operating chamber, a pumping mechanism discharging the fluid from the operating chambers to the reservoir chamber, and temperature responsive means controlling the amount of the fluid transferred from the reservoir chamber to the operating chambers by means of opening and closing the fluid return openings in response to changes of ambient temperature.
In the above-mentioned conventional fluid coupling device, however, both the fluid transfer to the operating chambers and the fluid discharge therefrom which are accompanied by the movement of the valve plate cannot be smoothly and precisely accomplished. Therefore, the transmitting output torque has a relatively large hysteresis curve produced between the increasing process (shown in FIG. 3 by lines d, e and f) and in the decreasing process, and in an extreme case it may be difficult to precisely control the transmitting output torque in the decreasing process in the three steps.